Estrogen raises HDL cholesterol levels - an effect that may account for the favorable association between hormone replacement therapy (HRT) and coronary heart disease (CHD) risk in postmenopausal women. Several lines of evidence suggest that genetic factors also influence HDL levels, although the precise genes involved have not yet been determined. Recent evidence from the Estrogen Replacement and Atherosclerosis (ERA) trial (N = 309) indicates that certain allelic variants in the estrogen receptor-alpha (ER-alpha) gene are associated with more than a twofold increase in estrogen's effects on HDL cholesterol. Women with the favorable genotype (ca. 20% of women) experienced a 26% increase in HDL with HRT compared with a 13% increase observed in the remaining women. However, this trial was too small to determine if these effects translate into an angiographic or clinical benefit. The Heart and Estrogen Replacement Study (HERS) was a large (N = 2763) randomized clinical endpoint trial of HRT for secondary prevention of CHD. This clinical trial cohort provides an ideal opportunity to confirm or refute the associations and interactions observed in the ERA trial with respect to lipids, and extend these observations to other estrogen-sensitive intermediate endpoints and clinical disease outcomes. Therefore, we propose to genotype HERS women with respect to several ER-alpha polymorphisms, and to examine the relationship between these ER-alpha genotypes, HRT use, change in HDL, and risk for CHD events. We will also examine the effects of ER-alpha polymorphisms on other plasma lipids, C-reactive protein, bone mineral density, risk for venous thromboembolic events, stroke, fractures, and all-cause mortality. Use of data and specimens from HERS is an efficient means to study the clinical impact of ER-alpha polymorphisms and possible modulation of estrogen action. If there are common polymorphisms in the ER-alpha gene that modify estrogen's effects on HDL and possibly other domains of estrogen action, this information could improve patients' and clinicians' ability to assess risks and benefits of HRT use. In addition, this information could lead to fundamentally important new knowledge about mechanisms of estrogen action, regulation of HDL cholesterol, and pathogenesis of CHD.